You will most likely need to get a wireshark capture and then look at the telephony tab and get the flow so you can see how both a successful and an unsuccessful call are seen (or not) by 3CX. You will see the SIP servers communicating and then see the media server come into play which is how the RTP packets are handled (on a successful call). Likely comparing the calls you will see the absence of a media server; hence why no audio. The phone will ring, you will answer and on a bad call, will enjoy dead silence.

Take a look at the router and ensure that the SIP ALG is off. I had an instance a couple of weeks ago with an Arris cable modem and built-in wireless with router that while the firewall was down, and the SIP ALG was shown to be off, it still intermittently acted as the SIP ALG was on. The frustrating thing was that if you unplugged the device, it would come back and act normally for a while, but then would revert some time later. I finally had to escalate the issue to the cable company's 3rd tier support, who also verified the settings as being correct, but I had the good fortune of getting a tech who understood and agreed to change the modem out. Since then, all has been well. The SIP ALG will normally alter the IP addresses and will send the audio to an invalid IP. I do not know why, in my case, the modem worked sometimes and failed at others, but I could clearly see the issue in the wireshark capture.

I also agree that the firewall test is an indication and not an absolute. It always passed in my case, but then the ALG never came into play.

What are youe planning on swapping out the ADSL router with? If it is provided by your ISP I'd forgo that as well, and purchase a that is modem to work well with VoIP. Do you have the Belkin ADSL in bridgemode and the Watchguard acting as firewall/router? Maybe if you tried repliacting the issue take a look at the wireshark, and server logs that would give you more if an indicator of where the issue lies. Post the logs just insert xxx.xxx.xxx.xxx's for ips etc...

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I find that the combination modem/routers, while providing simplicity, are not always the best choice when it comes to something more complicated than "surfing the web". If they are supplied by the provider then they usually give your provider access to your network/settings for troubleshooting (great for the majority), while being sparse on features. Some will also include in-built VoIP features that while they may not be used at present, can cause conflicts with 3CX. For those reasons I avoid them, and given the choice, recommend that to others. Of course it isn't always possible.

That said, if you have a chance to try a stand-alone modem, and a router, then it gives the chance to bypass the router and eliminate it as the problem.

If you list the model number of the modem/router (and perhaps the provider you are dealing with), someone else on the forum may have experience with it. It may simply be one setting that needs to be changed.

There is also the (remote) possibility, that your Belkin is faulty, given, that it works on occasion. On occasion, a complete reset, and re-provision, can help.